Cellular antioxidants are crucial for reducing oxidative stress and preventing neuronal death. A recently elucidated pathway to induce antioxidant enzymes involves transcriptional activation through the antioxidant-responsive element (ARE) (Itoh et al., Mol. Cell. Biol. 24:36-45, 2004; Gong et al., Antioxid. Redox Signal. 4:249-257, 2002). In this case, electrophilic agents induce a set of genes encoding “phase 2” enzymes, including hemeoxygenase-1 (HO-1), NADPH quinine oxidoreductase 1, and γ-glutamyl cysteine ligase (γ-GCL, also known as γ-glutamate cysteine ligase or γ-glutamyl cysteine ligase [γ-GCS]). These enzymes provide efficient cytoprotection, in part, by regulating the intracellular redox state (Itoh et al., Mol. Cell. Biol. 24:36-45, 2004; Gong et al., Antioxid. Redox Signal. 4:249-257, 2002).
The ARE is a cis-acting element essential for transcriptional activation of phase 2 genes by electrophiles (Itoh et al., Mol. Cell. Biol. 24:36-45, 2004; Gong et al., Antioxid. Redox Signal. 4:249-257, 2002). The transcription factor Nfr2 complexes with Maf family proteins to transactivate the ARE. Under basal conditions, the cytosolic regulatory protein Keap1 binds tightly to Nrf2, retaining it in the cytoplasm (Itoh et al., Mol. Cell. Biol. 24:36-45, 2004; Gong et al., Antioxid. Redox Signal. 4:249-257, 2002). In this regard, the action of Keap1 is analogous to that of IκB, preventing activation and translocation of the transcription factor NF-κB (Itoh et al., Mol. Cell. Biol. 24:36-45, 2004; Gong et al., Antioxid. Redox Signal. 4:249-257, 2002). In the case of Keap1, electrophiles make a Michael adduct with critical cysteine residues in this regulatory protein, causing the liberation of Nrf2 and allowing it to translocate into the nucleus (Itoh et al., Mol. Cell. Biol. 24:36-45, 2004; Gong et al., Antioxid. Redox Signal. 4:249-257, 2002).
Among the phase 2 enzymes, HO-1 has attracted special attention because of its therapeutic effects against neurodegenerative diseases (Maines and Panahian, in Hypoxia: From Genes to the Bedside, eds. Roach et al. (New York: Kluwer), 2001, pp. 249-272; Stocker et al., Science 235:1043-1046, 1987).
HO-1 oxidatively cleaves heme to biliverdin, forms CO, and releases the chelated Fe2+ (Maines and Panahian, in Hypoxia: From Genes to the Bedside, eds. Roach et al. (New York: Kluwer), 2001, pp. 249-272). Bilirubin (a reduction product of biliverdin) serves as a potent radical scavenger (Stocker et al., Science 235:1043-1046, 1987) and protects neuronal ells against oxidative stress at nanomolar concentrations (Dore et al., Proc. Natl. Acad. Sci. USA 96:2445-2450, 1999). Studies using gene-knockout and transgenic mice have confirmed the biological significance of HO-1 as a cellular antioxidant (Poss and Tanegawa, Proc. Natl. Acad. Sci. USA 94:10925-10930, 1997). HO-1 has been proposed to play an obligatory role in endogenous defense against oxidative stress, because cells from HO-1−/− mice are highly susceptible to oxidative insults (Poss and Tanegawa, Proc. Natl. Acad. Sci. USA 94:10925-10930, 1997). The significance of HO-1 in terms of drug development against neurodegenerative diseases is based on two facts: (i) HO produces several antioxidative compounds, including biliverdin and bilirubin (Dore et al., Proc. Natl. Acad. Sci. USA 96:2445-2450, 1999), and (ii) the induction of HO-1 can be regulated by various compounds (Satoh et al., Eur. J. Neurosci. 17:2249-2255, 2003). Thus, it has been proposed that an inducer of HO-1 in neurons could represent an efficient neuroprotective compound (Maines and Panahian, in Hypoxia: From Genes to the Bedside, eds. Roach et al. (New York: Kluwer), 2001, pp. 249-272; Dore et al., Proc. Natl. Acad. Sci. USA 96:2445-2450, 1999; Satoh et al., Eur. J. Neurosci. 17:2249-2255, 2003).